STACEE Science   

Gamma-Ray Astrophysics:

      Astronomy is the study of the universe by the detection of light. However, visible light corresponds to
only a small fraction of the electromagnetic spectrum--with an energy range extending from below
infrared to above ultraviolet, both of these being invisible to the human eye. Modern astronomers also
observe the sky at even lower energies (radio waves) and higher energies (x-rays and gamma rays).
Astrophysical gamma rays can have energies higher than those produced by the largest particle
accelerators.

      Today, a large part of the gamma-ray energy range remains unexplored. No telescope has ever been
built that can detect gamma rays in this unopened window. Observations in this window may be key
to understanding the mechanisms for high energy particle acceleration in some of the most powerful
astrophysical sources, including extremely dense objects known as pulsars and extremely bright
objects known as active galactic nuclei (AGN). Additionally, observations of AGNs, which are at distances
comparable to the size of the universe will yield information important to cosmology, the study of the
birth and evolution of the universe.

      To access the unexplored gap in the gamma-ray energy range for the first time, a group of physicists
and astronomers have built a new experiment called the Solar Tower Atmospheric Cherenkov Effect
Experiment (STACEE). STACEE is unusual in that much of the experiment was built for an entirely different
purpose: the collection of sunlight for solar energy studies. STACEE islocated at the National Solar Thermal
Test Facility (NSTTF), a solar energy research center located at Sandia National Laboratories in
Albuquerque, NM. This facility contains an array of 212 large mirrors called heliostats, each about 20 feet
by 20 feet in area. The heliostats are used during the day to track the sun and focus its light, but at night
the STACEE group uses them to collect Cherenkov radiation, the flashes of blue light that result when
gamma rays enter the earth's atmosphere. Using these large mirrors as part of the detector, the STACEE
group has built a gamma ray telescope with enormous collecting area for a very limited cost. The large
collection area is what makes it possible to explore the new energy range for gamma rays.

Accessing the unopened window

    Gamma-ray astrophysics is a young and dynamic field at the cross-roads of particle physics and
astrophysics. Recent discoveries by both space-borne and ground-based instruments have
revolutionized the field and led to an explosion of interest around the world. Perhaps the most
exciting challenge facing the field is the quest to explore a region of gamma-ray energy (or
wavelength) which has not been studied by any instrument before. Currently gamma-ray energies
between 20 and 250 GeV are inaccessable to both spaceborne detectors, such as the EGRET
experiment aboard NASA's Compton Gamma-Ray Observatory, and ground-based air cherenkov
detectors, such as the Whipple Observatory. This unopened window represents an energy regime
where many interesting phenomena are expected to occur.


    The need for a new detector in the energy range from 20 to 250 GeV is highlighted by comparing the
low-energy (EGRET) and high-energy (Whipple and similar detectors) all-sky maps of point
sources. While EGRET has seen more than 150 sources at energies up to 20 GeV, ground-based
experiments have detected only six convincing gamma-ray sources above 300 GeV. Something very
interesting is clearly happening in the gamma-ray spectrum of these sources in the gap between 20
and 250 GeV. It is believed that observations of sources in the gap will provide important evidence
concerning the acceleration mechanisms of the most energetic objects in the Universe, including
rotating neutron stars (pulsars), remnants of exploded stars (supernovae), gamma-ray bursts, and
distant, but intense, active galactic nuclei (quasars). Observations between 20 and 250 GeV may also
provide a direct probe of the diffuse intergalactic infrared radiation, which in turn may have profound
implications for the cosmological structure of the Universe. The importance of building experiments
to explore this energy range is highlighted by the existence of competing devices such as CELESTE
as well as both ground-based instruments, such as VERITAS, and space-borne missions, such as GLAST.
These major instruments will greatly expand our knowledge of the high energy Universe. For more
details on the scientific goals of gamma-ray astrophysics, please consult the list of STACEE scientific papers
and related links.

What are the primary science objectives for STACEE?

These are some of the central scientific questions that STACEE has been designed to address:

(1) What is the source of energy that powers brilliant blazars and other AGN found at the center of
galaxies?

(2) What is the maximum energy for photons emitted from AGN jets? What does this tell us about
the particles in the beam?

(3) At what energies are gamma rays from distant sources attenuated? What can this tell us diffuse
intergalactic infra-red and optical emission?

(4) What are the highest energies seen for pulsed emission from gamma-ray pulsars? What does
this tell us about the location of particle acceleration in these sources?

(5) Are supernova remnants sources of gamma rays at 100 GeV? Does this confirm our expectation
that SNR are likely sources of high energy cosmic rays?

(6) Are there other sources, including over 40 unidentified EGRET sources, that may also be detected
in the energy range from 50 to 250 GeV which has thus far remained completely unexplored?

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